The invention is based on a multiple-circuit brake system for motor vehicles such as a tractor-trailer. A brake system of this type is known from German Offenlegungsschrift 29 37 657. In this known brake system, the multiple-circuit braking value transducer is a two-circuit compressed-air brake valve, which is combined with an electric potentiometer in order to actuate a third brake. As a result, however, this known brake system has many parts and is expensive.